A six month old girl with no significant medical history was admitted to hospital for progressive cyanosis of recent onset refractory to oxygen therapy. There were no detectable cardiac murmurs. Chest X-ray showed an irregular left posterior parahilar infiltration. Echocardiography showed dilatation of the left atrium and ventricle but no cardiac malformation. The diagnosis of pulmonary arteriovenous fistula was suspected. Chest CT scan, magnetic resonance imaging and pulmonary angiography demonstrated the arteriovenous fistula in the lower lobe of the left lung. It was much bigger than the appearances of the chest X-ray suggested. After the left lower lobectomy, the cyanosis completely disappeared. Progressive cyanosis refractory to oxygen therapy without any apparent cardiac or pulmonary disease is strongly suggestive of pulmonary arteriovenous fistula. Surgical treatment is curative whereas the spontaneous outcome of this condition may be lethal.